In general, in a coke oven, various kinds of operational problems occur due to the progress of aging. Among such operational problems, “sticker” is a very serious operational problem that it is not possible to discharge manufactured coke from a coke oven. Since “sticker” occurrence forces change in a schedule for manufacturing coke, the amount of produced coke becomes decreasing and the life of the coke oven becomes shortened due to induced damage to the oven body. Therefore, decreasing the frequency of occurrence of “sticker” is given first priority in the operation.
A mechanism by which “sticker” occurs is roughly described as follows. The operation of a general chamber-type coke oven involves carbonizing a coal blend which has been charged into a carbonizing chamber to form a coke cake sequentially from the oven wall side due to heat transferred from a combustion chamber adjacent to the carbonizing chamber. Here, usually, since a coke cake shrinks due to carbonization, a gap (hereinafter, referred to as “clearance”) is formed between the oven wall and the outer surface of the coke cake. Formation of the clearance facilitates to discharge (extrude) the coke cake from the coke oven.
However, since an insufficient amount of shrinkage of a coke cake does not form a sufficiently large clearance, “sticker” occurs due to increased frictional resistance between the oven walls and the outer surface of the coke cake when the coke cake is extruded. Also, in the case where irregularity of the oven wall surface is large, “sticker” occurs due to increased frictional resistance between the oven walls and the outer surface of the coke cake. The irregularity of the oven wall surface is increased as a result of the abrasion and removal of oven wall bricks, an increase in the amount of carbon adhered to the oven walls, and so forth due to the aging of the coke oven. Therefore, frequency of occurrence of “sticker” inevitably increases due to the aging of a coke oven. In consideration of such a background, in case of operating of an aging coke oven, various countermeasures are implemented in order to decrease the frequency of occurrence of “sticker”.
A moisture-coal operation can be mentioned for an example of countermeasures aimed at decreasing the frequency of occurrence of “sticker”. The moisture-coal operation involving using a coal blend of which the moisture content is not actively decreased from the content (about 8 mass % to 14 mass %, although it depends on season and weather) when the coal blend is piled in a coal yard. The moisture-coal operation is widely used as the simplest and effective method. Increasing the moisture content of a coal blend makes the bulk density of a charged coal blend decrease and there is an increase in clearance or the like, thereby reducing frictional resistance between the oven walls and the surface of a coke cake when the coke cake is extruded. At the result thereof, it is possible to decrease the frequency of occurrence of “sticker”.
As a specific example of the method described above, Patent Literature 1 discloses a technique involving carbonizing a coal blend in a coke oven after the moisture content of the coal blend has been controlled by a coal-moisture-controlling apparatus. The technique involves determining the target moisture content of a coal blend necessary to achieve desired clearance on the basis of the relationship derived in advance between the moisture content of the coal blend and clearance, and controlling the heat input to a coal-moisture-controlling apparatus so that the total moisture content of the coal blend at the exit of the coal-moisture-controlling apparatus is the target moisture content. Such controlling decreases the frequency of occurrence of “sticker”.
In addition, Patent Literature 2 discloses a technique involving adding water locally to a coal in a coal tower and charging the coal into a carbonizing chamber via a larry car. The technique makes clearance increase due to an increase in the shrinkage ratio of coke in a part of the coal having an increased moisture content existing locally in the carbonizing chamber. The increase of clearance results in a decrease in the frequency of occurrence of “sticker”.
The conventional techniques described above have a common technical feature. The feature is increasing the moisture content of coal to be charged into a coke oven to form a clearance with an increased shrinkage ratio when carbonization is performed.
On the other hand, a blast furnace operation recently involves blowing pulverized coal into a blast furnace in order to decrease the amount of coke used. The operation needs coke having relatively higher strength, in particular, coke excellent in terms of drum strength which is determined by using a drum strength test method prescribed in JIS K 2151 is necessary. The blast furnace requires sufficient gas permeability and liquid permeability so as to progress the reducing reaction of iron ore efficiently and stably. In case of insufficient coke strength, there occurs a problem of a decrease in gas permeability and liquid permeability in a hollow space called a “raceway” which is formed in front of a tuyere and the lower part of the blast furnace due to the collision of coke particles.
Techniques for improving coke strength are largely classified into three groups in terms of processes in which they are used, that is, pretreatment techniques, blending techniques, and carbonizing techniques. In particular, pretreatment techniques are considered to be important, because the techniques makes it possible to design equipment so that there is no limitation on the productivity of a coke oven without an increase in the costs for coal blend. Such pretreatment techniques are classified mainly into the following two groups in terms of the approach to coke strength.
(1) A technique for improving the charged bulk density of a coal blend (hereinafter, referred to as “technique (1)”)
(2) A technique for homogenizing a coal blend (hereinafter, referred to as “technique (2)”)
The technique (1) is intended to decrease the number of pore defects which influence coke strength. The technique (1) involves mechanically compacting a coal blend to improve charged bulk density and charging the coal blend into a coke oven so as to reduce the interparticle space of the coal. The technique (1) results in an improvement in coke strength. Specific examples of the technique include a method of charging coal briquettes partially, a stamping method, and a method of decreasing the moisture content of a coal blend in order to decrease the interparticle adhesiveness of the coal to improve the charged bulk density (refer to Non Patent Literature 1). However, a process in which the moisture content of a coal blend is decreased by using a moisture-controlling apparatus or a preheating apparatus is introduced into an operation of many coke ovens.
In contrast, the technique (2) is intended to increase the strength of a portion of coke having the lowest strength. Since coal is fundamentally composed of textures having different properties in terms of various thermal and mechanical properties, coal is very inhomogeneous. Naturally, the structure of coke, which is manufactured from such inhomogeneous coal, is also inhomogeneous. Generally, the strength of a brittle material such as coke is described on the basis of a weakest link model and determined by the strength of a portion having the lowest strength in the material. Therefore, since the strength of coke is homogenized by homogenizing the structure of the coke, the technique (2) makes it possible to improve the strength of the entire coke.
Examples of a method for the technique (2) include a method in which the particle size of coal is controlled (refer to Non Patent Literature 1). The method of controlling the particle size of coal is basically intended to homogenize the structure of coke by finely pulverizing coal. Also, a method is known which is intended to homogenize the structure of coke by treating coal with a coal-mixing machine such as a drum mixer in order to increase the degree of mixing of the coal (refer to Non Patent Literature 2). However, it has been clarified by conventional research that, without being treated with a coal-mixing machine, a coal blend which is used in a coke-making process is sufficiently mixed, for example, at connection parts of a belt conveyer in a transportation process (refer to Non Patent Literature 2). Therefore, in many coke plants, consideration is given to homogenize the structure of coke without using a coal-mixing machine nowadays.